The present invention relates to the production of papermaking pulp from wood chip feed material, and particularly to mechanical refining and chemi-mechanical refining.
Efforts have been ongoing for decades to improve mechanical refining techniques (including chemi-mechanical refining) for producing papermaking pulp from wood chip feed material with decreasing specific energy requirements. A significant advance toward this objective was achieved by the present inventor in the mid 1990's, by the development of the “RTS” process, as described in U.S. Pat. No. 5,776,305, granted on Jul. 7, 1998, for “Low-Resident, High-Temperature, High-Speed Chip Refining. This development was directed to the relationship between chip pre-heat environment and high consistency primary refiner conditions, whereby a window of pre-heat residence time, pre-heat saturated steam temperature (pressure) and high disc refining speed produced a noteworthy reduction in specific energy required to achieve commercial strength properties, while retaining satisfactory optical properties.
A significant further development by the present inventor is the “RT Pressafiner” pretreatment, upstream of preheating and primary refining, as described in International Patent Application No. PCT/US98/14710, filed Jul. 16, 1998, for “Method of Pretreating Lignocellulose-Contaning Feed Material”. According to the RT Pressafiner development, chip feed material received, for example, from an atmospheric pre-steaming bin, is first conditioned at elevated temperature and pressure for a controlled period of time, and then highly compressed at elevated temperature and pressure, whereupon the pretreated chips may be conveyed directly into the preheater portion of a primary refiner, or retained in an atmospheric bin until subsequent feeding to the preheater of a primary refiner.
The combination of the RT Pressafiner pretreatment with the RTS primary refining, produces an exceptionally energy efficient mechanical refining system, due largely to the significant extent of axial separation of the fibers in the chips fed to the primary refiner. Although the RT Pressafiner pretreatment method and apparatus has been highly effective in producing axially separated fibers (i.e., separated along the grain), there appears to be an upper limit on axial separation of about 25-30 percent of the total chip mass.